DevTools: Remove redundant default destructor and forward declarations

Problem:
- Default destructors (and constructors) are in `.cpp` files. This
  prevents the compiler's optimizer from inlining them when it thinks
  inlining is appropriate (unless LTO is used).
- Forward declarations can prevent some optimizations, such as
  inlining of constructors and destructors.

Solution:
- Remove them or set them to `= default` and let the compiler handle
  the generation of them.
- Remove unneeded forward declarations.

Userland/DevTools/Inspector/RemoteProcess.cpp

@@ -8,37 +8,11 @@
 #include "RemoteObject.h"
 #include "RemoteObjectGraphModel.h"
 #include "RemoteObjectPropertyModel.h"
-#include <InspectorServer/InspectorClientEndpoint.h>
-#include <InspectorServer/InspectorServerEndpoint.h>
-#include <LibIPC/ServerConnection.h>
 #include <stdio.h>
 #include <stdlib.h>
 
 namespace Inspector {
 
-class InspectorServerClient final
-    : public IPC::ServerConnection<InspectorClientEndpoint, InspectorServerEndpoint>
-    , public InspectorClientEndpoint {
-    C_OBJECT(InspectorServerClient);
-
-public:
-    virtual void handshake() override
-    {
-        greet();
-    }
-
-    virtual ~InspectorServerClient() override { }
-
-private:
-    InspectorServerClient()
-        : IPC::ServerConnection<InspectorClientEndpoint, InspectorServerEndpoint>(*this, "/tmp/portal/inspector")
-    {
-        handshake();
-    }
-
-    virtual void dummy() override { }
-};
-
 RemoteProcess* s_the;
 
 RemoteProcess& RemoteProcess::the()
@@ -54,10 +28,6 @@ RemoteProcess::RemoteProcess(pid_t pid)
     m_client = InspectorServerClient::construct();
 }
 
-RemoteProcess::~RemoteProcess()
-{
-}
-
 void RemoteProcess::handle_identify_response(const JsonObject& response)
 {
     int pid = response.get("pid").to_int();